Monoethylene glycol is used as a raw material in the manufacture of polyester fibres, polyethylene terephthalate (PET) plastics and resins. It is also incorporated into automobile antifreeze liquids.
Monoethylene glycol is typically prepared from ethylene oxide, which is in turn prepared from ethylene. In order to produce ethylene oxide, ethylene and oxygen are passed over an epoxidation catalyst, for example, a silver oxide catalyst, typically at pressures of 10-30 bar and temperatures of 200-300° C., producing a product stream comprising ethylene oxide, carbon dioxide, ethylene, oxygen and water. The amount of ethylene oxide in the product stream is usually between about 0.5 and 10 weight percent. The product stream is supplied to an ethylene oxide absorber and the ethylene oxide is absorbed by a recirculating solvent (absorbent) stream containing mostly water. After absorption, the aqueous ethylene oxide stream is sent to a stripper in order to separate the ethylene oxide. The ethylene oxide leaves the top of the stripper as a concentrated aqueous ethylene oxide stream.
In one well-known process, ethylene oxide is then catalytically reacted with carbon dioxide to produce ethylene carbonate. The ethylene carbonate is subsequently hydrolysed to provide ethylene glycol. Reaction via ethylene carbonate significantly improves the selectivity of ethylene oxide conversion to monoethylene glycol compared to the known process wherein ethylene oxide is reacted with a large excess of water to form ethylene glycol in a non-catalytic process.
The catalytic reaction of ethylene to ethylene oxide usually takes place in the presence of a moderator, which controls the performance of the epoxidation catalyst. Commonly used moderators include monochloroethane or dichloroethane. The use of these organic chloride moderators leads to the presence of organic chloride contaminants in the ethylene oxide product stream that is supplied to the ethylene oxide absorber. These organic chloride contaminants are absorbed in the ethylene oxide absorber, are present in the stream supplied to the ethylene oxide stripper and are present in the concentrated aqueous ethylene oxide stream taken from the top of the ethylene oxide stripper.
If the ethylene oxide is catalytically converted to monoethylene glycol via ethylene carbonate, the presence of the organic chloride contaminants in the concentrated ethylene oxide stream can lead to problems. The organic chloride contaminants can react with hydrolysis catalysts such as potassium carbonate to produce inorganic chloride contaminants (e.g. potassium chloride). Therefore, the contaminants can cause degradation of the hydrolysis catalysts and can also lead to a build-up of inorganic chloride. The inorganic chloride can start to precipitate and may cause chloride stress corrosion.
To avoid build-up of inorganic chloride it is usual to remove a portion of the catalyst via a catalyst bleed in the catalyst recycle loop (and thereby also remove inorganic chloride contaminants). So that expensive catalyst is not lost, it is also usual to recover catalyst from the catalyst bleed so that it can be reused. This is a relatively expensive process, so it is desirable to limit the quantity of catalyst bleed.
The present inventors have sought to provide an improved process wherein the requirement for a catalyst bleed is reduced.